I have a home build amp, similar to the Fender Twin, 4 6L6GC tubes but with a switchable rectifier, bias and high/low option. See my drawing. It's only the power amp, the preamp didn't fit in this chassis and I'm making that separate, but I can still use it like this. And it sounds quite good too.

But the plates have a small red spot on them and I can't figure out why at that level of dissipation.

I have set it at 74mA per side with the bias voltage at -35v, plate voltage about 412v and still have the redness. That's the extreme of the bias pots I have. I can go down to -22v but that would clearly be to much current.

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diyaudio is against your rights. I do not have the right to be forgotten and after asking the issue has been ignored, therefore I'm leaving (peacefully) for this reason. I stand up for my rights. So long and thanks for all the Fish

The 6L6 wants to be run in class AB, not class A at this plate voltage... I'd adjust the values of the resistors that go to ground from the bias pots *up* in ohms so you can get more negative voltage on the grids.

The other thing is to drop the screen voltage and current a bit.

As far as I can tell you are pretty close to proper operating voltages and currents, but these are import tubes, and the best thing to do is to adjust them empirically and not rely upon any mfrs specs.

You *do have negative voltage* there? Just checking...

Also, I don't see why you switch bias points AND switch to a cathode bias as well... I guess it does change the tone a bit, but why not just keep the same negative bias and switch in the cathode resistor of appropriate value??

_-_-bear

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_-_-bear http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]

Problem solved. I was using the OT shunt method and tried the cathode resistor method for measuring current. I measured 54.8mV on the cathode. That's my method from now on. I'm not sure if it's my meter or the method but it explains the redness.

"Also, I don't see why you switch bias points AND switch to a cathode bias as well... I guess it does change the tone a bit, but why not just keep the same negative bias and switch in the cathode resistor of appropriate value??"

It's the only way I know to go from fixed bias to cathode bias. It drops the output power a bit and makes the response spongier and loose-more for blues and rock than for jazz or metal playing-as far as I read...

I have not tried it without the NFB, but the circuit I have lets me get to about 0.8%. The Blackface Twin of the 60's has about 12%. Much cleaner and using the pot I can go from 20% to 0.8%-clean to mean...